graphics/swfmill: unbreak with clang 4.0

gSWFParseXML.cpp:19:10: error: ordered comparison between pointer and zero ('iconv_t'

(aka '__tag_iconv_t *') and 'int')
          if (cd < 0) {
              ~~ ^ ~

gSWFWriteXML.cpp:15:10: error: ordered comparison between pointer and zero ('iconv_t'

(aka '__tag_iconv_t *') and 'int')
          if (cd < 0) {
              ~~ ^ ~

Reported by: antoine (via exp-run)